Many Organic compounds are acids or bases (or both) Many Organic compounds undergo acid-base reactions

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1 Objective 4 Intro to Reactivity 1: identify acids and bases using Lewis definition. Use curved arrows to show how base reacts with acid. Relate strength to pk a. Determine direction of equilibrium. Use pk a table to estimate pk a of acid based on structure.

2 Many Organic compounds are acids or bases (or both) Many Organic compounds undergo acid-base reactions Acid-base (proton transfer) reactions are very fast Dimethyl amine is a base and smells like dirty socks. Neutralize with acid to get rid of smell. Formic acid is in ant venom and stings when you are bitten by an ant. Use a base, e.g., baking soda, for relief.

3 Many Organic compounds are acids or bases (or both) Many Organic compounds undergo acid-base reactions Tawny crazy ant uses formic acid as fire ant venom antidote CEN, 3/3/14, p. 44 Butyric acid smells terrible. Use a base to get rid of smell. Amino acids change charge (and properties) depending on ph.

4 Many Organic compounds are acids or bases (or both) Many Organic compounds undergo acid-base reactions Acid-base (proton transfer) reactions are very fast Lab 2: Do an acid-base extraction to separate a mixture: You have a mixture of a weak acid and weaker acid. Add a base that reacts with one acid but not the other è Separate weak acid from weaker acid.

5 Acid-Base (H + Transfer) Reactions Are Very Common In Organic Chemistry Bronsted-Lowry definition: Acids are proton (H + ) donors Bases are H + acceptors Every acid has a partner (conjugate) base. (Remove acidic H) HCl + H 2 O --> H 3 O + + Cl - acid base conjugate acid conjugate base of of Lewis definition: Acids are electron pair acceptors (electrophile) Bases are electron pair donors (nucleophile) Electron pairs ===> curved arrows

6 HCl is a strong acid Organic acids are weak acids Use K a and pk a to determine acid strength Generic Acid: HA <==> H + + A - pk a = - log K a Ka = [H+ ][A - ] [HA] Which organic compound is the stronger acid? Give reasons. Draw the conjugate base of each compound. Which conjugate base is stronger? Give reasons.

7 Table 3.1 pk a of common compounds (Klein, p. 100) 1. Large pk a ==> weak acid 2. Every acid has a partner (conjugate) base 3. Acids are listed from strongest to weakest 4. Bases are listed from weakest to strongest 5. Conjugate base of a weak acid is strong 6. Conjugate acid of weak base is strong 7. PREDICT: An acid reacts with any base below it ==> A Weak Acid Reacts with a Strong Base

9 Acids and Bases Objective: determine relative acid strength a. Rank the following compounds in order of acid strength Rank each number in sequence from strongest to weakest. b. Which of the above compound(s) behave like a base?

10 Acetic acid is an acid with 4 H s. Which H is acidic? See pk a table. pk a of H 1 pk a of H 2

11 Acids and Bases Objective: determine the conjugate base of an acid. What is the conjugate base of CH 3 COOH?

12 Amino acids have at least 2 acidic H s. Which H is the most acidic, H 1 or H 2 or H 3? Use pk a table to estimate acid strength. pk a of H 1 pk a of H 2 pk a of H 3

13 Rubbing alcohol reacts with a base. Which H reacts? In other words, which H is more acidic, H 1 or H 2? Use pk a table to estimate acid strength. Which base would you use? Draw the product. pk a of H 1 pk a of H 2

14 Estimate base strength from structure. Note: Base accepts a H +. So basic atom has a lone pair. Draw conjugate acid. Estimate pk a of conjugate acid. Stronger acid (pk a big or small?) means (stronger or weaker?) base. pk a of H 1 pk a of H 2

15 Curved Arrows show Bonds Breaking and Forming Start (arrow tail) at electron pair donor/lewis base/ NUCLEOPHILE and End (arrow head) at electron pair acceptor/lewis acid/ ELECTROPHILE 1. Lone pair on O forms σ bond to H 2. Too many bonds on H so H-Cl σ bond breaks to form lone pair on Cl

16 Acids and Bases Objective: use curved arrows to show bonds breaking and forming in a reaction. CH 3 COOH reacts with NaOH. Which curved arrow shows this reaction? Then, draw the products of the reaction. This reaction is an equilibrium reaction. Are there more products or reactants?

17 Objective: use curved arrows to show bonds breaking and forming in a reaction Draw the product(s) of the following reaction. Use numbers to represent curved arrows to show bonds breaking and forming.

18 Objective: use curved arrows to show bonds breaking and forming in a reaction Draw the product(s) of the following reaction. Use numbers to represent curved arrows to show bonds breaking and forming. More practice: Klein, Problem 3.44

19 1. For a strong acid, K a is big and pk a is. For a weak acid, K a is and pk a is big. 2. a. Rank the following acids in order of strength: HCl, H 2 O, acetic acid, ethanol, ethane, ethylene. b. Which acid in part a reacts with ammonia (pk a of NH 4 + = 9.3)? c. Is the conjugate base of a strong acid strong or weak?

20 Use pk a to Determine the Position of Equilibrium Predict the products of this reaction: CH 3 COO - + CH 3 OH à Use curved arrows to show bonds breaking and forming. Will there be more products or more reactants? More products More reactants What is the position of the equilibrium? More products More reactants More practice: Klein, Problem 3.52

21 Use pk a to Determine the Position of Equilibrium Predict the products of this reaction: Use curved arrows to show bonds breaking and forming. Will there be more products or more reactants? What is the position of the equilibrium? More reactants More practice: Klein, Problem 3.52

22 Application: Use pk a To Predict Charge at a Specific ph (see Lab 2) In stomach (ph 2), aspirin charge = In blood (ph 7.4), aspirin charge = In stomach (ph 2), glycine charge = In blood (ph 7.4), glycine charge =

23 Application: Use pk a To Predict Charge at a Specific ph At ph = pk a ====> equal [ ] of acid and conjugate base At ph > pk a ====> more conjugate base than acid At ph < pk a ====> more acid than conjugate base ph Buffer region endpoint 1/2 way point Volume of base added

24 Lab 2: Use Acid-base Reaction to Separate an Acid-baseneutral Mixture Flow Chart Acid-base-neutral mixture 1. Dissolve in solvent 2. Add acid?????????? What solvent would you use? Example: Klein, Problem 3.41

25 How Does Structure Affect Acid Strength? See Klein, Chapter 3.4 (Or How Do You Estimate pk a Without a pk a Table?) If the conjugate base is stable, acid is strong. Stability of base depends on: 1. Atom (want (-) charge on more EN atom) Which acid is stronger? 2. Resonance stabilization 3. Inductive effects (EN atom attracts charge and spreads it out) 4. Orbital (charge closer to nucleus is more stable, e.g., compare hybrid orbitals)

26 Objective: ID the most acidic proton. (Hint: see pk a table. If not in the pka table, draw conjugate base. More stable base means stronger acid.)

27 Use curved arrows to show how B: - reacts with each H. Draw the structure of each product. Include resonance structures as needed. B: - 1 Conjugate base Y is more stable than X because.

28 Use curved arrows to show how B: - reacts with each H. Draw the structure of each product. Include resonance structures as needed. B: -

29 Use curved arrows to show how B: - reacts with each H. Draw the structure of each product. Include resonance structures as needed. B: -

30 CEN, 4/2/12, p. 8 New Class of Superbases Cyclopropenimine is a very strong base (3.5 orders of magnitude higher than that of comparable guanidines) and catalyzes reactions up to 800 times faster. Explain why Cyclopropenimine is a strong base. Identify the bonds that are broken in the reactants and bonds that are formed in the product.

31 Lewis base Lewis acid Electron pair donor Electron pair acceptor, e.g., H + (-) charge (+) charge (-) pole (δ-) (+) pole (δ+) Nucleophile (Nu: - ) Electrophile (E + ) Electron Source Electron Sink lone pair, e.g., OH - electron deficient species π bond less EN atom in polar bond Curved arrow starts at e - pair donor and ends at e - pair acceptor. Identify each compound as a Lewis acid or Lewis base. A B C D E

32 Lewis acids (Electrophiles): HCl, (CH 3 ) 3 C +, C bonded to Br Lewis bases (Nucleophiles): pi bond, OH - lone pair Curved arrow starts at e - pair donor and ends at e - pair acceptor. Use curved arrows to show bonds breaking and forming ==> show how reactants form products. Follow Bonding and Structure Rules. Draw the product of each reaction (4 total). More practice: Klein, Problem 3.39

33 Bonding, Structure, Shape, Acid-Base Reactivity Review 1. a. Each C in C 3 H 8 is tetrahedral, with hybridization, with σ bonds and π bonds. b. A sp 2 hybridized C has a geometry with σ bonds and π bonds. An example is. 2. a. How do you know when a molecule has resonance structures? b. What is the difference between a resonance structure and an isomer?

34 Bonding, Structure, Shape, Acid-Base Reactivity Review a. Identify the acidic proton (E + ) in each compound: C 2 H 6 C 2 H 4 C 2 H 2 CH 3 OH (CH 3 ) 2 CO CH 3 COOH b. Draw the conjugate base. c. Rank the acids in order of strength. Give reasons for your ranking.

35 1. Draw the conjugate base of the following acids. Which acid is stronger? 2. Identify the Lewis acid and Lewis base. Then, use numbers to represent curved arrows to show bonds breaking and forming and how the product is formed.

36 Identify the acid and base. Then, draw curved arrows showing a proton transfer reaction. Draw the products and predict the position of equilibrium.

Many Organic compounds are acids or bases (or both) Many Organic compounds undergo acid-base reactions

Many Organic compounds are acids or bases (or both) Many Organic compounds undergo acid-base reactions Objective 4 Intro to Reactivity 1: identify acids and bases using Lewis definition. Use curved arrows to show how base reacts with acid. Relate strength to pk a. Determine direction of equilibrium. Use